$results = db_select('users', 'u') ->fields('u',array('name','uid','mail')) ->execute();
$options = array();
foreach ($results as $result) {
$options[$result->uid] = $result->name.' | '.$result->mail;
}
unset($options[array_search(0, $options)]);
$form['user'] = array(
'#type' => 'select',
'#title' => t('Select User'),
'#options' => $options,
'#default_value'=>('select a value'),
'#states' => array(
'visible' => array(
':input[name="data_set"]' => array('value' => 'education'),
 ),
 ),
);
$form['submit']=array(
'#type'=>'submit',
'#value'=>t('Expire')
);

$options = array('_none' => '-select item-') + $options; //Places an extra option at the start

// Use the array key, not the value.
  '#default_value' => '_none',